asm-generic: make atomic_add_unless a function

atomic_add_unless is a macro so, bad things happen if the caller defines
a local variable named c, just like like the local variable c defined by
the macro. Thus, convert atomic_add_unless to a function. (bug triggered
by net/ipv4/netfilter/ipt_CLUSTERIP.c: clusterip_config_find_get calls
atomic_inc_not_zero)

Signed-off-by: Mathieu Lacage <mathieu.lacage@inria.fr>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>